CVE-2026-72404

In the Linux kernel, the following vulnerability has been resolved:

tipc: fix UAF in cleanup_bearer() due to premature dst_cache_destroy()

TIPC UDP media bearer teardown calls dst_cache_destroy() on its

replicast caches before calling synchronize_net() to wait for

concurrent RCU readers (transmitters) to finish:

static void cleanup_bearer(struct work_struct *work)

{

...

list_for_each_entry_safe(rcast, tmp, &ub->rcast.list, list) {

dst_cache_destroy(&rcast->dst_cache);

list_del_rcu(&rcast->list);

kfree_rcu(rcast, rcu);

}

...

dst_cache_destroy(&ub->rcast.dst_cache);

udp_tunnel_sock_release(ub->sk);

synchronize_net();

...

}

This is highly buggy because dst_cache_destroy() immediately frees the

per-CPU cache memory (free_percpu()) and releases the cached dst

entries without any synchronization.

If a concurrent transmitter (e.g., tipc_udp_xmit()) is running on another

CPU under RCU protection, it can call dst_cache_get() concurrently,

leading to:

1. Use-After-Free on the per-CPU cache pointer itself (crash).

2. "rcuref - imbalanced put()" warning if it attempts to release a

dst that was concurrently released by dst_cache_destroy().

Furthermore, calling kfree(ub) immediately after synchronize_net() without

closing the socket first (or waiting after closing it) leaves a window

where a concurrent receiver (tipc_udp_recv()) could start after

synchronize_net(), access ub, and suffer a UAF when kfree(ub) runs.

To fix this, we must defer dst_cache_destroy() and kfree(ub) until after

we have ensured that no more readers can see the bearer/socket and all

existing readers have finished:

1. Defer rcast entry destruction (both dst_cache_destroy() and kfree())

to an RCU callback using call_rcu_hurry().

Using call_rcu_hurry() ensures the dst entries are released quickly.

2. Release the bearer socket using udp_tunnel_sock_release() (stops

new receive readers).

3. Call synchronize_net() to wait for all outstanding RCU readers

(both transmit and receive) to finish.

4. Now that it is safe, call dst_cache_destroy() on the main bearer

cache, and free ub.

Note: 3) and 4) can be changed later in net-next to also use

call_rcu_hurry() and get rid of the synchronize_net() latency.